1. Field of the Invention
The present invention relates to a phase detector which is used in a frequency-modulation receiver (FM receiver) of a type wherein an original modulation signal is detected from a complex baseband signal generated through orthogonal demodulation of a frequency modulated signal (FM signal), and more particularly, to an improvement of a phase detection method for detecting an original modulation signal independent of the amplitude of the received signal and a phase detector and an FM receiver to which the phase detection method is applied.
2. Description of the Related Art
In conventional FM receivers, there is a type in which an original modulation signal is detected from a complex baseband signal generated through orthogonal demodulation of an FM signal. In this type, the center frequency of the FM signal is equal to the local frequency. Compared with a superheterodyne receiver (another type of the conventional FM receivers), the first-mentioned type has various advantages which includes the following.
a. An image suppressing filter is unnecessary. PA1 b. A baseband filter which determines a channel selectivity can be formed in an integrated circuit (IC). PA1 c. A detection circuit can be formed in an IC since signal detection is performed in the baseband signal. PA1 (A) When the baseband signal is in the first quadrant, the output is Q when the 1-bit binary value is "0", and -I when the output of the delay step is "1"; PA1 (B) When the baseband signal is in the second quadrant, the output is -I when the 1-bit binary value is "0", and -Q when the 1-bit binary value is "1"; PA1 (C) When the baseband signal is in the third quadrant, the output is -Q when the 1-bit binary value is "0", and I when the 1-bit binary value is "1"; PA1 (D) When the baseband signal is in the fourth quadrant, the output is I when the 1-bit binary value is "0", and Q when the 1-bit binary value is "1".
Incidentally, the orthogonal demodulation may be carried out directly at a reception frequency (direct conversion reception method) or may be carried out at an intermediate frequency by lowering the reception frequency. Mobile communication apparatus such as a radio telephone set can be made small and light by employing the direct conversion reception method.
FIG. 17 shows an example of an FM receiver of this type. An FM signal Rcos (.omega.ct+.phi.) (where ##EQU1## m(t): modulation signal) supplied to an input terminal 101 is sent to mixers 102 and 103 where it is subjected to orthogonal demodulation with use of local signals cos .omega.ct and --sin.omega.ct having an angular frequency .omega.c equal to the intermediate frequency, and then sent to low-pass filters 104, 105 and amplifiers 106, 107 to generate complex baseband signal components I(t)=(R/2) cos.phi.(t) and Q(t)=(R/2) sin.phi.(t), respectively.
The baseband signal component I is differentiated at a differentiator 108 and multiplied by the baseband signal component Q at a mixer 111, while the baseband signal component Q is differentiated at a differentiator 109 and multiplied by the baseband signal component I at a mixer 110. A subtractor 112 subtracts the output of the mixer 111 from the output of the mixer 110 to obtain the modulation signal m(t). The above processes are expressed as follows. PG,4 ##EQU2##
However, the signal thus detected includes a coefficient (R.sup.2 /4) whose value is proportional to the square of the amplitude R of the received signal. As a result, the detected signal may fluctuates according to the fluctuation in the amplitude of the received signal caused by fading or the like.
Accordingly, in the example of FIG. 17, the complex baseband signal components I and Q are squared at mixers 113 and 114 and then added together at an adder 115 to generate a signal corresponding to the coefficient (R.sup.2 /4). The coefficient signal is supplied to a divider 116 to divide the above detection output (R.sup.2 /4)m(t) so as to obtain a normalized output m(t). The normalized output is sent via a low-pass filter 117 to an output terminal 118.
Typically, the outputs of the amplifiers 106 and 107 are converted at A/D converters (not shown) into digital values for digital signal processing (DSP) in the subsequent stages. This is advantageous because digital type differentiators 108, 109, multipliers 110, 111, 113, 114, subtractor 112, adder 115, etc can be operated with higher accuracy compared with those of analog type. Further, this is advantageous because signals in other modulation methods can be detected with the same hardware configuration if software is changed.
For the dividing operation performed at the divider 116, four dividing methods are known, i.e., reciprocal ROM method, logarithm calculation method, subtraction shift method and convergence division method (Refer to "Signal Processor and its Applications", Shoukou-dou, by Rikio Maruta and Takao Nishiya, pp.38-41). However, a dividing method capable of performing dividing operation with satisfactory high accuracy and speed has not been developed. The absence of satisfactory dividing method is a serious obstacle which has prevented an FM receiver of this type requiring a division circuit from being developed.
Conventional FM receivers of this type cannot absorb the fluctuations in the detection output caused by fluctuations in the amplitude of the FM signal, thus making it difficult to implement an FM receiver having a high signal reception accuracy.